It is well known in the art that all amplifiers distort an inputted signal. The distortion becomes greater as the power levels are increased. When amplifier compression and group delay distortion is introduced into a single information carrying channel, or when the amplifier is exposed to multiple input signals, intermodulation (IM) products are introduced. These IM products must be kept low because they interfere with the system or with other users. Methods have therefore been developed to reduce these types of distortion.
There are two types of distortion introduced by an amplifier. The first type of distortion, referred to as linear distortion, is related to the frequency dependent gain and phase response of the amplifier. The phase response is not the absolute phase value, but rather the part differing from the linear phase, i.e., the time delay in the amplifier. The second type of distortion, referred to as non-linear distortion, is produced by the non-linear gain-to-input power in the amplifier. The linear part of the distortion plays a significant role in wide band applications.
The distortion caused by single carrier power amplifiers can be dealt with by using feedback techniques such as cartesian feedback. The main drawback with using feedback is the narrow bandwidth within which the distortion can be reduced because of stability constraints.
The wideband amplifier is becoming more attractive as a carrier power amplifier because of the improved systems flexibility it offers over ordinary multiple single carrier power amplifiers using passive filter combination techniques at their outputs. In wideband systems, there are two main methods for dealing with distortion: predistortion; and feed-forward. Predistortion means that the non-linear and/or the linear part of the distortion introduced by the main amplifier is inversely modeled at the input of the amplifier, thereby making the total, ideally, equal to a frequency independent constant. To reduce the linear distortion, a simple filter is needed; and to reduce the non-linear distortion, a "curve bender" needs to be used. However, both methods have to be adapted to overcome aging, temperature and sample differences.
The feed-forward technique consists basically of two independent steps. The first step is to extract the distortion introduced by the main amplifier on the signals to be amplified. This is referred to as extracting an error signal. The second step is to inject this error signal in anti-phase and time aligned at the output of the feed-forward amplifier to thereby cancel out the distortion.
The main differences between the feed-forward technique and the predistortion method is that in the feed-forward technique the distortion is extracted rather than modelling the distortion causing element, making the feed forward technique more sample independent. The two steps can be repeated at the output of the feed-forward amplifier system to further suppress distortion components. The performance of the feed-forward technique is dependent upon the ability to add rotated signal vectors correctly in anti-phase and equal amplitude. This process determines how well a distortion component can be extracted or suppressed. The ability to control these variables, i.e., gain and phase, are therefore of crucial importance in feed-forward amplifier systems.
Several methods for controlling the extraction of distortion and/or injection of the extracted distortion in anti-phase have been introduced in the past. There are two main methods of how to control a feed-forward amplifier system. The first method uses the actual distortion caused by the main amplifier to control the setting of the gain and phase controls. This is possible since it equals the first step in the feed-forward process. In this case, the error signal content is minimized at the output. Second method uses a known distortion simulating signal which is injected in the amplifier path and minimized at the output thereby also reducing the distortion introduced by the main amplifier. The term distortion shall be understood to mean any signals present in the output of a device which were not C present at the input.
U.S. Pat. No. 4,389,618 is an example of the first method for controlling a feed-forward amplifier system in which the system correlates the extracted distortion with the distortion present at the output. The correlation yields a setting of the gain and phase of the extracted distortion, i.e., the error signal, which is subtractively combined with the output. The patent also shows the addition of a third loop to reduce the signal components from the output. This decreases the correlation score between signal components at the output and the remaining signal components, due to linear distortion in the main amplifier, in the extracted error signal. The correlation score between distortion components has to be higher than the correlation score between signal components in order to yield the best setting of the gain and phase of the subtractively combined error signal.
U.K. Patent No. 2 167 256A discloses a similar system which includes a non-automated third loop. The reduction in signal component correlation score due to the third loop is not sufficient for a wideband feed-forward amplifier system due to the linear distortion introduced by the main amplifier. To overcome this problem, U.K. Patent Application No. 2 244 881 uses multiple input controls, which to a high degree will cancel out the linear part of the distortion emanating from the main amplifier.
With a multiple input system with one input loop for every input channel, all linear distortion can theoretically be removed. The extracted error signal would then only contain non-linear distortion which can be correlated with the output signal to yield a non-interfered setting of the amplitude and phase control of the subtractively combined error signal.
The number of required input loops is governed by the linear distortion, exhibited over the used frequency range by the main amplifier and will determine the amount of the remaining unwanted signal components in the error signal. In a simple correlation technique, it is desirable for the remaining signal components to be less than the extracted non-linear distortion in order to achieve easy control of the second loop. To avoid over specification of the linear part of the distortion in the main amplifier, the amount of input loops can increase rapidly in a wideband system, with an increase in the number of control loops and other components to follow.
U.S. Pat. No. 4,580,105 is an example of the second method for controlling a feed-forward amplifier system where a narrow band distortion simulating pilot tone is injected into the system. The pilot tone is extracted in a narrow band pilot receiver at the output of the amplifier. The pilot tone is detected and minimized using a decreasing step size circuit algorithm thereby reducing the distortion introduced by the main amplifier. The drawback with injected pilot tones is that a pilot tone cannot be given an arbitrary frequency, but is governed by the input signals to be amplified in the feed-forward amplifier. The pilot tone must be placed in the same frequency range as the input signal in order to be helpful. Since the pilot has no special labeling or modulation, the control scheme is sensitive to other narrowband signals on the same frequency, such as various signals within the transmitter system.